The principal study carried out in the past year has been the characterization of rabbit-mouse hybridoma cells secreting rabbit immunoglobulin chains. Secreted products of several cell lines have been isolated and structural characterization is in progress. Results obtained to date indicate that the rabbit secreted products (either H or L chain) are normal both structurally and serologically. An unusual mode of covalent association has been discovered between a rabbit H chain and a mouse L chain in one hybridoma product. Several characteristics of the cells secreting these molecules have also been examined, particularly with a view obtaining appropriate mutants for further fusion studies between various hybridomas.